1. Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 17: Powder-Metal Processing and Equipment
Presentation slide for courses, classes, lectures et al.
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2. Chapter Outline Introduction Production of Metal Powders
Compaction of Metal Powders
Sintering
Secondary and Finishing Operations
Design Considerations
Process Capabilities
Economics of Powder Metallurgy
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3. Introduction
Raw materials for metals and alloys are in a molten state (casting) or in solid form (metalworking)
Powder metallurgy (PM) process involves metal powders compacted into desired shapes and sintered to form a solid piece
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4. Production of Metal Powders
Powder-metallurgy process consists of:
Powder production
Blending
Compaction
Sintering
Finishing operations
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5. Production of Metal Powders: Methods of Powder Production
The choice of producing metal powders depends on the requirements of the end product
The microstructure, bulk and surface properties, chemical purity, porosity, shape, and size distribution of the particles depend on the particular process used
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6. Production of Metal Powders: Methods of Powder Production
Atomization
Involves a liquid-metal stream produced by injecting molten metal through a small orifice
Stream is broken up by jets of inert gas or air or water known as gas or water atomization
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7. Production of Metal Powders: Methods of Powder Production
Atomization
Size and shape of the particles formed depend on the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics
In centrifugal atomization, the centrifugal forces break up the stream and generate particles
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8. Production of Metal Powders: Methods of Powder Production
Reduction
The reduction of metal oxides uses gases, such as hydrogen and carbon monoxide, as reducing agents
Electrolytic Deposition
Used either aqueous solutions or fused salts
Carbonyls
Metal carbonyls are formed by letting iron or nickel react with carbon monoxide
Reaction products are decomposed to iron and nickel
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9. Production of Metal Powders: Methods of Powder Production
Comminution
Involves crushing milling in a ball mill, or grinding of brittle or less ductile metals into small particles
A ball mill is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls
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10. Production of Metal Powders: Methods of Powder Production
Mechanical Alloying
Powders of two or more pure metals are mixed in a ball mill
Under the impact of the hard balls, the powders fracture and bond together by diffusion
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11. Production of Metal Powders: Particle Size, Shape, and Distribution
Particle size is controlled by passing the metal powder through screens of various mesh sizes, called screening
Other methods for particle-size analysis are:
Sedimentation
Microscopic analysis
Light scattering
Optical methods
Suspending particles
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12. Production of Metal Powders: Particle Size, Shape, and Distribution
Particle Shape
Particle shape is described in terms of aspect ratio or shape factor
Aspect ratio is the ratio of the largest dimension to the smallest dimension of the particle
Shape Factor
A measure of the ratio of the surface area of the particle to its volume
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13. Production of Metal Powders: Particle Size, Shape, and Distribution
Size Distribution
Affects the processing characteristics of the powder
Distribution of particle size is given in terms of a frequency-distribution plot
Properties of metal powders that affect their behavior in processing are:
Flow properties
Compressibility
Density
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14. Production of Metal Powders: Blending Metal Powders
Blending (mixing) powders is the next step in powder- metallurgy processing
Powder mixing must be carried out under controlled conditions in order to avoid contamination or deterioration
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15. Production of Metal Powders: Blending Metal Powders
Hazards
Metal powders can be explosive due to high surface area–to-volume ratio
Precautions include:
Grounding equipment
Preventing sparks and avoiding friction as a source of heat
Avoiding dust clouds, open flames, and chemical reactions
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16. Compaction of Metal Powders
Compaction is the step where the blended powders are pressed into various shapes in dies
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17. Compaction of Metal Powders
Purposes of compaction are to obtain the required shape, density and particle-to-particle contact
Pressed powder is known as green compact
Density depends on the pressure applied
Higher the density of the compacted part, the higher are its strength and elastic modulus
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18. Compaction of Metal Powders
May be necessary to use multiple punches to ensure that the density is more uniform throughout the part
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19. Compaction of Metal Powders: Equipment
Compacting pressure required depends on the characteristics and shape of the particles, method of blending and lubricant
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20. Compaction of Metal Powders: Equipment
Hydraulic presses with capacities as high as 45 MN, can be used for large parts
Press selection depends on part size and the configuration, density requirements, and production rate
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21. Compaction of Metal Powders: Isostatic Pressing
Green compacts is subjected to hydrostatic pressure to achieve more uniform compaction and density
In cold isostatic pressing (CIP), the metal powder is placed in a flexible rubber mold
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22. Compaction of Metal Powders: Isostatic Pressing
Green compacts is subjected to hydrostatic pressure to achieve more uniform compaction and density
In cold isostatic pressing (CIP) the metal powder is placed in a flexible rubber mold
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23. Compaction of Metal Powders: Isostatic Pressing
In hot isostatic pressing (HIP), the container is made of a high-melting-point sheet metal and the pressurizing medium is a high-temperature inert gas
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24. Compaction of Metal Powders: Isostatic Pressing
The HIP process is used to produce superalloy components for the aircraft and aerospace industries
It also is used:
To close internal porosity
To improve properties in superalloy and titanium-alloy castings for the aerospace industry
As a final densification
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25. Compaction of Metal Powders: Isostatic Pressing
Advantages of hot isostatic pressing are:
Produces fully dense compacts of uniform grain structure and density
Handling larger parts
Limitations of HIP:
Wider dimensional tolerances
Higher equipment cost and production
Small production quantities
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26. Compaction of Metal Powders: Isostatic Pressing
EXAMPLE 17.1
Hot Isostatic Pressing of a Valve Lifter
Figure shows a valve lifter for heavy-duty diesel engines
Produced from a hot-isostatic-pressed carbide cap on a steel shaft
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27. Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes
Powder-injection Molding
Also called metal-injection molding
Very fine metal powders are blended with a 25 to 45% polymer or a wax-based binder
Mixture undergoes a process similar to die casting
Advantages of powder-injection molding are:
Complex shapes
Good dimensional tolerances
High production rates
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28. Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes
EXAMPLE 17.2
Mobile Phone Components Produced through Metal Injection Molding
Powder-metal components for mobile phones to achieve a flip-open feature
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29. Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes
Forging
Products from powder forging (PF) are fully dense, good surface finish, good dimensional tolerances, and a uniform and fine grain size
Rolling
Also called roll compaction
Metal powder is fed into the roll gap in a two-high rolling mill and compacted into a continuous strip
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30. Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes
Extrusion
Powders is compacted by extrusion whereby the powder is encased in a metal container and hot extruded
Pressureless Compaction
The die is gravity filled with metal powder and the powder is sintered directly in the die
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31. Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes
Spray Deposition
It is a shape-generation process die
Basic components are
An atomizer
A spray chamber with an inert atmosphere
A mold for producing preforms
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32. Compaction of Metal Powders: Punch and Die Materials
Selection depends on the abrasiveness of the powder metal and the number of parts to be produced
Close control of die and punch dimensions is essential for proper compaction and die life
Large clearance will allow the metal powder to enter the gap
Die and punch surfaces must be polished for improved die life and overall performance
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33. Sintering
Sintering is the process whereby green compacts are heated in a furnace to below the melting point but high enough to allow bonding (fusion) of the individual particles
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34. Sintering
Strength of the bond between the particles depends on the complex mechanisms of diffusion of:
Plastic flow
Evaporation of volatile materials in the compact
Recrystallization
Grain growth
Pore shrinkage
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35. Sintering Continuous-sintering furnaces have 3 chambers:
Burn-off chamber
High-temperature chamber
Cooling chamber
The sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering
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36. Sintering Mechanical Properties
Affecting mechanical properties are temperature, time, and processing history
Porosity cannot be avoided completely due to voids remaining after compaction and gases evolve during sintering
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37. Sintering Mechanical Properties
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38. Sintering Mechanical Properties
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39. Sintering Mechanical Properties
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40. Secondary and Finishing Operations
Further improvement for the properties of sintered PM products are:
Coining and sizing
Preformed and sintered alloy-powder
Subject to finishing operations
Reduce porosity by impregnating PM components with a fluid
Infiltration
Electroplating
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41. Design Considerations
Certain design principles that should be followed:
Simple and uniform shape of the compact
Provision must be made for ejection of the green compact
PM parts made with acceptable dimensional tolerances
Part walls should not be less than 1.5 mm thick
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42. Design Considerations
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43. Design Considerations
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44. Process Capabilities Process capabilities of powder metallurgy are:
A technique for making parts from high-melting-point refractory metals
High production rates
Good dimensional control
Availability of a wide range of compositions
Capability of impregnation and infiltration
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45. Process Capabilities Limitations of PM are: High cost of metal powder
High cost of tooling and equipment
Limitations on part size and shape complexity
Mechanical properties are lower
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46. Economics of Powder Metallurgy
Powder metallurgy can produce parts at net or near- net shape
Eliminate many secondary manufacturing and assembly operations
High initial cost of punches, dies, and equipment
Economical for quantities over 10,000 pieces and labor costs would not be high
Reduces scrap
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47. Economics of Powder Metallurgy
CASE STUDY 17.1
Powder Metallurgy Parts in a Snowblower
Collection of PM parts in a commercial snowblower
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